Many LEDs based on semiconductor materials without using phosphors are known. For example, InGaN semiconductor materials are used for blue LEDs and AlGaInP semiconductor materials are used for red LEDs. Green LEDs are produced based on InGaN semiconductor materials and amber LEDs are produced based on AlGaInP semiconductor materials. Of these, InGaN-based blue LEDs are known to have an external quantum efficiency of at least 60% but InGaN-based green LEDs was reported to have an external quantum efficiency of 30% or below. The lower external quantum efficiency of InGaN-based green LEDs is attributed to numerous physical factors, such as lattice mismatch caused by an increased amount of gallium (Ga). Red LEDs based on AlGaInP semiconductor materials reach a quantum efficiency of at least 50%, whereas amber LEDs based on the same materials were reported to have a quantum efficiency up to 15% due to their poor characteristics caused by an increased amount of aluminum (Al).
FIG. 1 graphically shows the quantum efficiencies of a green single-color LED and an amber single-color LED. Referring to FIG. 1, the external quantum efficiencies of the green and amber LEDs in the wavelength range of 530-610 nm cannot reach high levels due to the inherent problems of InGaN and AlGaInP semiconductor materials. This phenomenon is called “green window” or “amber gap” of LEDs.
It was reported that AlGaInP-based amber LEDs undergo a drastic shift in emission spectra and an increase in applied current with increasing temperature, resulting in lower efficiency than InGaN-based blue LEDs. InGaN-based green LEDs were also reported to show a drastic drop in efficiency resulting from an increase in applied current. This phenomenon is called “droop”, which limits their use as high-luminance LEDs.
In attempts to overcome such problems, phosphor-converted LEDs (pc-LEDs) that use near-UV LEDs or blue LEDs as excitation sources to cause phosphors to emit light were developed and reported.
In recent years, research and development has been conducted on phosphor-converted green or amber LEDs as single-color pc-LEDs that use near-UV LEDs as excitation sources because invisible UV light is transmitted through the single-color pc-LEDs. Upon energy transfer to the phosphors, however, a large energy difference is caused by the Stark shift and thus a large energy loss occurs, which is called “energy deficit”. Such energy loss leads to low efficiency of the LEDs.
On the other hand, in the case of LEDs using blue LEDs as excitation sources and nano/micro quantum dots or phosphor powders to emit green or amber light, blue light from the blue LEDs leaks without exciting the phosphor powders. This light leakage causes poor color purity of the LEDs. In order to overcome the problem of light leakage, a combination of a blue LED and an amber fluorescent ceramic substrate was reported as an amber LED in 2009. The amber LED uses a ceramic plate phosphor to completely convert blue light from the blue LED to amber light. Due to the complete light conversion, the amber LED was reported to achieve a color purity of at least 95. However, incomplete transparency of the ceramic plate phosphor causes partial reflection or loss of blue light from the blue LED, and as a result, the photoconversion efficiency of the amber LED still remains low. The production of the ceramic plate phosphor requires techniques under high-temperature and high-pressure conditions, which may cause partial chemical decomposition or oxidation of the phosphor.
In order to overcome the problems encountered in the fabrication of single-color LEDs using ceramic plates, there arises a demand to develop phosphor-converted single-color LEDs using nano/micro powder-based pc-LEDs whose production processes are well known in the art. The fabrication of phosphor-converted single-color LEDs using nano/micro powder-based pc-LEDs requires complete absorption and blocking of light emitted from blue LEDs by the nano/micro powders. Therefore, the amounts of the phosphors added to silicon matrices should be larger than are needed. The phosphors at high concentrations tend to aggregate. This aggregation may give rise to scattering/reflection of both light excited by the blue LEDs and light emitted from the phosphors, leading to optical loss. Further, light emitted from the phosphors can be re-absorbed, resulting in a drastic decrease in luminous efficacy. Under these circumstances, there is a need to develop a phosphor-converted single-color LED that can emit a single color due to its high color purity, luminance and efficiency despite the use of a nano/micro powder-based pc-LED without using a phosphor at a high concentration.